Semiconductor devices containing integrated circuits (ICs) or discrete devices are used in a wide variety of electronic devices. The IC devices (or chips, or discrete devices) can include a miniaturized electronic circuit that has been manufactured in the surface of a substrate of semiconductor material. The circuits are composed of many overlapping layers, including layers containing dopants that can be diffused into the substrate (called diffusion layers) or ions that are implanted (implant layers) into the substrate. Other layers are conductors (polysilicon or metal layers) or connections between the conducting layers (via or contact layers). IC devices or discrete devices can be fabricated in a layer-by-layer process that uses a combination of many steps, including imaging, deposition, etching, doping and cleaning. Silicon wafers are typically used as the substrate and photolithography is used to mark different areas of the substrate to be doped or to deposit and define polysilicon, insulators, or metal layers.
One type of semiconductor device, a high-electron-mobility transistor (HEMT), is a field-effect transistor incorporating a junction between two materials with different band gaps (i.e. a hetero-junction) as the channel instead of a doped region (as is generally the case for MOSFET devices). HEMTs can be used in integrated circuits as digital on-off switches. HEMT transistors can operate at higher frequencies than ordinary transistors and can be used in many high-frequency products, including, for example, mobile phones.